The invention relates to production of negative continuous-tone image by means of a photographic material comprising at least one silver halide emulsion layer the characteristic curve of which has a defined sagging course.
Reproduction of continuous-tone original is conventionally carried out by first producing a negative continuous-tone image from which a positive continuous-tone image or a screened image is then prepared which serves as an original for the subsequent photomechanical process (layer which are hardened upon exposure, pigment paper, transfer film, etc.) for the production of the printing form. The continuous-tone original may be a black and white or multicolor reflection copy or transparency. In the case of a color image, the negative continuous-tone images are color separation negatives if they have been obtained using color separation filters. These continuous-tone negatives are usually produced by exposure in a reproduction camera, in other words by optical exposure, but they may also be produced by contact.
Since the contrasts of originals used for reproduction, i.e. the differences in density between the lightest and darkest important parts of the image ("light" and "shadow") in practice vary over a wide range of about 1.00 to 3.00, especially in the case of transparent multicolor originals (color reversal diapositives) whereas the continuous-tone negatives produced from them (color separation records) are required to have a substantially narrower range of variation in this respect (prestandardisation) or may even be required to have a fixed value for the contrast with narrow tolerances if they are to be used by conventional methods (standardised negatives), the reproduction process requires the use of several films, generally three, with stepped gradation .gamma. (.gamma. about 0.50, 0.90 and 1.40).
As is well known, the use of several types of films for the same purpose entails numberous disadvantages both for the manufacturer and for the consumer since the different films cannot be produced so accurately that they will be sufficiently uniform in all their essential properties, and there may be unavoidable differences, e.g. in the sensitivity, the filter factors, the color of the silver image, the uniform fog density, the so-called residual coloring (due to the sensitizer or the antihalation layer) the dimension stability, etc.. Moreover, it is very inconvenient both for the manufacturer and the supplier and especially the photographer in the dark room to keep several films in reserve, and in the dark room especially it requires great care to ensure that they will not be mixed up. Moreover, the production of three different films is not efficient from the manufacturer's point of view. To this is added the fact that the production of light sensitive layers which have a very flat density curve extending in a straight line right up to high densities, which is necessary at least for the flattest of the three films mentioned above, involves considerable difficulties in the preparation of the emulsions.
Assuming that the exposed continuous-tone materials will be processed automatically in processing machines which for reasons of efficiency can only be operated with a particular development time, then continuous-tone images (color separation records) having a standardised contrast can no longer be produced by the methods hitherto employed and consequently their subsequent treatment by conventional methods is also impossible or at least extremely difficult since the subsequent operations must be carried out manually with variation of the development time.
Although in principle it is known how to produce standardised or prestandardised continuous-tone images even with processing machines since the gamma value or gradation of the material can be controlled by the color of the light used for exposure, the process employed is not suitable for multicolored reproduction because in the production of color separation records which, as is well known, are obtained with narrow-band blue, green and red filters, it is not possible to vary the color of the light for the purpose of controlling the gradation. For multicolor reproduction, therefore, the production of standardised color separation records using processing machines remains an unsolved problem in the present state of the art. The same applies to the production of prestandardised color separation records and, even more so to the production of standardised color separation records with the aid of a processing machine, if only one separation film is used for all the originals having the contrast ranges specified above.
Even the reproduction of black and white originals in which the above mentioned material whose gradation can be varied with the color of the light is used and standardised or prestandardised image can be produced, still entails difficulties and disadvantages which lie especially in the choice of the correct color of light (exposure with two color filters in succession with predetermined exposure time) and in the complicated technique used for exposure, which requires careful attention and time (calculations, readings of tables and graphs for adjustment of the exposure times). If this work is carried out with the aid of one of the known technical instruments, again increased technical expenditure and therefore cost is involved.
It is among the objects of the invention to provide processes for the production of negative continuous-tone images, which as a result of suitable choice of the sensitometric properties of the copying materials used, are suitable for automatic, mechanical processing methods.